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A049201
If n is not composite, a(n) = n followed by 1; if n is composite, a(n) = concatenation of prime factors of n.
1
1, 11, 21, 31, 22, 51, 23, 71, 222, 33, 25, 111, 223, 131, 27, 35, 2222, 171, 233, 191, 225, 37, 211, 231, 2223, 55, 213, 333, 227, 291, 235, 311, 22222, 311, 217, 57, 2233, 371, 219, 313, 2225, 411, 237, 431, 2211, 335, 223, 471, 22223, 77, 255, 317, 2213
OFFSET
0,2
PROG
(Python)
from sympy import factorint
def a(n):
if n == 0: return 1
f = factorint(n)
if n == 1 or sum(f.values()) == 1: return int(str(n)+'1')
return int("".join(str(p)*e for p, e in f.items()))
print([a(n) for n in range(53)]) # Michael S. Branicky, Jul 21 2021
CROSSREFS
Sequence in context: A109686 A077522 A308232 * A068633 A225297 A123849
KEYWORD
nonn,base,easy
EXTENSIONS
More terms from Michel ten Voorde
a(22) onward corrected by Sean A. Irvine, Jul 21 2021
STATUS
approved